EVP Asymmetric Encryption and Decryption of an Envelope

Encryption and decryption with asymmetric keys is computationally expensive. Typically then messages are not encrypted directly with such keys but are instead encrypted using a symmetric "session" key. This key is itself then encrypted using the public key. In OpenSSL this combination is referred to as an envelope. It is also possible to encrypt the session key with multiple public keys. This way the message can be sent to a number of different recipients (one for each public key used). The session key is the same for each recipient.